(1) FIELD OF THE INVENTION
This invention relates to a dry presensitized plate for use in making a dry planographic printing plate requiring no dampening water and having an overlying silicone rubber layer which acts as an ink repellent layer.
(2) DESCRIPTION OF THE PRIOR ART
There have been proposed various dry presensitized plates for use in making printing plates requiring no dampening water and having a silicone rubber layer which acts an ink repellent layer. For example, Japanese Patent Publication No. 54-26923 (British Patent 1,399,949) and Japanese Patent Publication No. 56-23150 disclose a presensitized plate comprising a support having provided thereon, in order, a photohardenable light-sensitive layer comprising a photopolymerizable composition and an overlying silicone rubber layer, and Japanese Patent Publication No. 55-22781 (British Patent 1,419,643 and German Patent 2,323,453) discloses one comprising a support having provided thereon, in order, a photohardenable light-sensitive layer comprising a photodimerizable resin and an overlying silicone rubber layer. Japanese patent application (opened to public inspection, hereinafter referred to as "OPI") No. 54-54702 discloses a presensitized plate comprising a light-sensitive layer of a photopolymerizable composition and an overlying silicone rubber layer comprising an organopolysiloxane and a vinyl triacyloxysilane. In last-mentioned presensitized plate, it is possible to reduce the content of ethylenic unsaturated monomer in the light-sensitive layer, and consequently to decrease deposition of printing paper powder, ink scum or dust on the light-sensitive layer and to produce non-image areas of the silicone rubber layer having increased adhesion strength to the light-sensitive layer and therefore non-image areas of the silicone rubber layer having high resistance to scratches during development or a press operation.
However, these types of presensitized plates have such properties that, depending on the circumstances, the silicone rubber layer is apt to adhere strongly to the photoadhesive light-sensitive layer as soon as it is provided on the light-sensitive layer. Therefore, when the presensitized plate is insufficiently developed after image-wise exposure, the silicone rubber layer in image areas is incompletely removed, so that the resulting dry printing plate is poor in fine dot reproduction. If the plate surface is too strongly rubbed during development in order to improve fine dot reproduction, the silicone rubber layer in non-image areas is damaged and the dry printing plate thus prepared is poor in printing properties, that is, it produces copies having background contamination and other defects. For avoiding this, it is necessary to lightly rub the plate surface with a soft developing pad, which prolongs the time required for development.